West Bengal's Salt Lake, several metro cities shrinking at alarming rate

The bustling Salt Lake area, on the eastern fringes of Kolkata, is shrinking at an alarming rate, a top scientist at the Geological Survey of India said. Salt Lake, mostly built by filling up water bodies and lakes, along with its adjoining areas in the metropolis, is experiencing groundwater depletion, which could be one of the reasons for land subsidence, Dr. Sandip Som, Director, Geological Survey of India, told PTI.

It is likely that parts of neighboring Kolkata are also shrinking, he said. "Data recorded over the past two-and-a-half years by the global positioning system (GPS) station at GSI's Salt Lake office have revealed that the area is subsiding at a rate of nearly 19-20mm per year.

"As the station is able to obtain coordinates within a 300-kilometre radius, we may also say that Kolkata is shrinking," Som said. The GSI is trying to find out the major "problematic" areas to be able to fix the issue, he insisted.

"Our calculations are minute with accuracy of plus or minus one millimeter for vertical and horizontal calculation. Land subsidence could be due to groundwater withdrawal or tectonic movement, we can't say for sure," he said.

Som also emphasized that metro cities across the country are dealing with the same predicament. "GPS stations set up across the country have revealed that the cities of Jaipur, Dehradun, Hyderabad and Bangalore are also shrinking... The data further showed that the entire Himalayan foothills and its adjoining areas are subsiding at varying rates.

"However, the Himalayan mountain range is rising. Also, Patna and Nagpur are rising, all due to varied movements of the tectonic plates," he said.

Of the cities facing subsidence, Jaipur and Salt Lake, in the neighborhood of Kolkata, are the worst affected, the GSI director maintained. Talking about the western part of the country, he said a "folding process" is currently underway.

Folds occur when a stack of originally flat surfaces, become bent or curved under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature gradient. "The West coast is also buckling. Researchers have found out through geophysical studies that a folding process is underway in the west coast of the country. We have two GPS stations, one at Thiruvananthapuram and another at Pune, which have also reported similar developments," he stated.

A total of 22 observatories have been set up in the country, including the one is Salt Lake, for geological and geo-physical mapping. "At the moment, 22 GPS and 10 broadband stations are operational. We are planning to establish a total of 35 permanent GPS stations along with broadband seismograms by which we will be able to measure the positional difference of a region. Our aim is to study the stress and strain (the earth's crust undergoes) across the country," Som added.

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